Pixels or Perish

Pyramid Scheme

I offer the illustration at right—along with the corresponding digital version below—as a case study. Population pyramids are a well-established tool in demography. In this case the pyramids show the age structure of the global human population over a 150-year period, according to estimates and projections published by the United Nations.

Tracing change over time is the main point of the illustration, yet this is notoriously hard to do in a static picture. Snapshots at 50-year intervals give some sense of the overall outcome: What begins as a pyramid evolves into an onion dome. But it’s not so easy to see how and why the shape is changing. One thing that’s not made explicit is how cohorts (groups of people born at about the same time) move upward through the age categories as time passes. Consider the bar at the base of the pyramid in 1950, which measures the number of people who were less than 5 years old in that year. The survivors of this group reappear in the 50-to-54-year-old bar in 2000, and a tiny sliver of centenarians remain in 2050. There’s nothing in the structure of the diagram to remind you that those three bars represent the same people.

No doubt a clever illustrator could improve the graphs in ways that would more clearly convey the basic facts of life: that births affect only the bottommost bar, and deaths shape all the rest. Showing more intermediate stages would certainly help. However, space on the page is always at a premium in a printed magazine.

The interactive version of the same illustration suggests some of the possibilities of more-dynamic visual media. Instead of looking at preselected snapshots, you can move through time, forward or backward, and watch the pyramid change shape as a result of births and deaths. Animated transitions emphasize the continuity of the human population, as cohorts migrate through the decades. With higher temporal resolution (5 years per step, rather than 50), it’s easier to spot noteworthy moments of transition. For example, it appears there was a sharp drop in worldwide fertility in about 1990; that’s when the sides of the pyramid grow noticeably steeper. And another landmark comes in about 2050, when each successive group of 0-to-4-year-olds ceases to be larger than the preceding cohort, so that the base of the “pyramid” becomes pinched. (Note: I am deeply interested in these demographic trends, but my aim here is to discuss the effectiveness of graphic presentations, not to debate the meaning or validity of the data.)